Note: Descriptions are shown in the official language in which they were submitted.
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Glass ceramic material for dental restoration and
method for producing same
Field of the invention
The invention relates to a glass ceramic for
dental restoration and to a method for its production.
Background of the invention
A glass ceramic of this type is known from
US 4,798,536. This patent describes a specific
porcelain material which is used as a tooth replacement
material. Porcelain as a tooth replacement material has
in principle been known for a long time, and is also
known to be relatively susceptible to breaking.
Therefore, metal substructures have generally been used
in order to obtain the desired strength.
US 4,798 536 has disclosed a porcelain material
which has breaking strength properties which are such
that it appears to be suitable for use as a dental
material on its own.
However, it has emerged that the forces and
loads which occur in the mouth may nevertheless impose
loads on the material which may cause it to break.
Summary of the invention
Working on the basis of this prior art, the
invention is based on the object of improving a glass
ceramic of the type mentioned in the introduction in
such a way that it is more resistant to breaking. A
further object of the present invention is to specify a
method for its production.
These objects are achieved by means of the
features of claims 1 and 9, respectively.
Advantageous configurations of the invention
are characterized in the subclaims.
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Detailed Description
The invention will now be described in more detail, by
way of example, with reference to the enclosed
diagrams, in which:
Fig. 1 shows a scanning electron microscope
image, one bar on the scale representing 10
micrometers, and
Fig. 2 shows an enlargement of the SEM image
from fig. 1.
A range of feldspar products, for example
Canadian or Norwegian feldspar, can be used as starting
materials to produce the semi-transparent ceramic, the
color of which is similar to that of teeth. Other
starting materials can also be used. The potassium-
sodium ratio is important when selecting the feldspar.
It is preferred to use potash feldspar, which in
mineral form always contains sodium. The ratio of Na20
to Kz0 should be less than 1 to 10. The glass ceramic on
which the invention is based, that is to say the end
product, corresponds to the following composition, in
percent by weight:
Material Percent by weight
SiOz 65 - 75
A1203 8 - 12
Na20 3 - 6
Ka0 8 - 11
Ca0 1 - 3
Ba0 0 - 2
Ce02 0.5 - 1
Ti02 0.2 - 0.5
823 0.5 - 2
The raw materials have been mixed in the form
of silicates, carbonates or oxides. The resultant
mixture of raw materials has been melted in a cascade
crucible at approximately 1500°C and a throughput of
approximately 2 kg/h. The residence time in the
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crucible was approximately one hour. The melt can also
be melted in a range from 1470°C to 1550°C and in
different form.
The glass melt has then been added dropwise
from the melting furnace directly into water,
solidifying spontaneously to form an amorphous glass in
grit form. After drying of the water-quenched glass
melt, the coarse-grained glass was milled in a ball
mill. As soon as the milled material stuck to the
milling drum, the dry milling operation was ended.
After the milling, the resultant glass powder was
screened through a screen of < 80 microns. The mean
grain size was approximately 20 microns.
To produce dental restoration products from
glass ceramic, the glass was then sintered and annealed
into the desired shapes and objects. These processes
are used in dental technology to produce the tooth
replacement.
In an alternative procedure, the glass ceramic
powder may also be dry-pressed and then converted into
a solid glass ceramic object by a sintering/annealing
firing operation. This procedure is advantageously used
to produce semi-finished products or blanks from which
an individual ceramic object, preferably for dental
restoration, is produced using CAD-CAM technology.
The material produced in accordance with the
above procedure is a leucite-reinforced ceramic which
can be used to produce fully ceramic crowns without
metal reinforcement. The leucite content of the new
glass ceramic is more than 90~. This information can be
found from the SEM images, since quantitative analysis
is generally inaccurate. The high coefficient of
thermal expansion of 19.0 * 10-6 K-1 also indicates a
high leucite content. The literature value of the
leucite is given as 20 to 22.
The crystals in US 4,798 536 are smaller than
35 microns, preferably smaller than 5 microns, and are
substantially in leaf form, with a length to width
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ratio of 1:1 to 1:3, and have principle dimensions in
two directions between 1 and 10 microns.
In the crystals according to the present
invention, a new needle-shaped, strongly matted
structure has been formed. The individual crystals are
needle- or column-shaped with a thickness of between
0.3 and 1.5 micrometers and have a length of between
7.5 and 20 micrometers. They are preferentially
oriented in groups in a star shape starting from a
nucleus as the center point, from which the leucite
crystals form along star-shaped paths. When the
amorphous glass is heated, the leucite crystals form in
the temperature range from 800 to 900°C. Definite
clouding of the originally transparent glass is
visually recognizable after a holding time of even
2 minutes. The columns are preferably 0.5 to
1 micrometer thick for a length of between 8 and
12 micrometers. The ratio of length to width is at
least 5:1 and may be up to 15:1. It is preferably in
the region of 10:1.
Test specimens were sintered from this glass
ceramic material with a high level of leucite crystals
in needle form produced in situ, these specimens were
sawn using a diamond saw and their three-point bending
strength was determined. The sawn specimens had a
bending strength of 200 MPa, and after a suitable
surface treatment, such as glazing, strengths of
between 300 and 350 MPa were achieved. The novel glass
ceramic material therefore has an improved breaking
strength and opens up new application areas for the use
of solid ceramic in dental technology, in particular
for metal-free restoration.
The composition of the glass now allows
simultaneous annealing and sintering. The
crystallization occurrs relatively spontaneously and
quickly, within a few minutes. After the sintering of
the glass powder, the needle-shaped crystals form in a
star-shaped arrangement. The conventional two-phase
production process is reduced to a single process step,
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in which sintering and crystallization take place
simultaneously. The crystal structure in needle or
fiber form which is formed increases the strength
compared to previous leucite forms.
The enclosed figures show SEM images in which
the glass ceramic material has a high concentration of
crystal needles matted together, with a diameter of
1 micron and a length of 10 microns. The three-point
bending strength of the leucite-reinforced glass
ceramic achieved is approximately twice as high as with
conventional leucite-reinforced ceramics.
The coefficient of thermal expansion for purely
leucite crystals is approximately 20 to 22 * 10-6 K-1.
With the glass ceramic according to the invention, it
is possible to achieve a value of 19 to 20 * 10-6 K-1,
which reflects the improved strength. The glass matrix
has a coefficient which is approximately 10 * 10-6 K-1.
The prior art reveals relatively inhomogeneous
leucite crystal distributions which form large surfaces
and therefore also large areas for forces to attack. In
the prior art, the leucite crystals are simply embedded
in amorphous materials and form islands of strength in
the amorphous glass phase. By contrast, the invention
provides long, thin crystals which are oriented in
numerous different directions and which, as can be seen
from the drawings, do not simply form islands of
strength in the amorphous glass phase, but rather
constitute strong, cohesive structures around the
nucleus at the center of the star, in which material
fractures in the microfracture range are reliably
halted even after short distances.
